The primary goal of the studies proposed is a better understanding of bone resorption, to be achieved by isolating the specific precursor to the osteoclast, and by demonstrating the differentiation of the isolated progenitor cells into functional osteoclasts. After the precursor of the osteoclast has been isolated, then the proliferation, migration/recruitment to bone, and differentiation of this cell can be studied, both under normal conditions, and in osteolytic disease states such as osteoporosis and periodontal disease. This could lead to the clinical management of osteoclastic populations and their functions. Osteoclasts are formed by the fusion of mononuclear precursors which originate extraskeletally. Recent evidence indicates that they are derived fom hemopoietic stem cells. Osteoclastic precursors will be identified by isolating various progenitor pools from normal bone marrow, using monoclonal antibodies to specific surface antigens (Thy 1.1) and fluorescence-activated cell sorting. The various precursor cell pools will be evaluated for their ability to differentiate into osteoclasts and resorb bone by three assay systems: 1) Precursor cells isolated from normal rat bone marrow will be tested for their ability to differentiate into functional osteoclasts in vivo, by transplanting them into inbred ia osteopetrotic recipient rats. The value of this system is that osteoclasts of donor origin can be morphologically distinguished from mutant cells. The cellular pools will be tested for their ability to cure the skeletal sclerosis in the ia rats. 2) Precursor cells will be combined with devitalized bone chips in diffusion chambers. The chambers will be implanted into recipient rats to test the ability of the progenitor cells to differentiate into osteoclasts and resorb bone in response to different bone substrates, but in the absence of other bone cells. 3) Progenitor cells will be co-cultured with fetal long bones in vitro to determine if the interaction of the progenitor cells with the bone substrate as well as bone-forming cells is necessary to initiate the differentiation of osteoclasts and bone resorption. The cumulative results of these assays should further define the precursor to the osteoclast.